(a) Field of the Invention
The present invention is directed to coating compositions for substrates such as paper and paperboard. More particularly, the invention relates to a coating composition containing poly(ethylene oxide) and an improved method for applying higher coating weights at standard coating operating conditions than would normally be obtainable. An improved coated paper is also provided.
(b) Related Art
Aqueous pigmented coatings are generally applied to paper to provide a superior surface for printing and other converting applications. These coatings are made up of pigments as the main ingredient with binders and various additives, which give the paper a fairly uniform surface on which printing ink can be applied. The desirable properties of the finished paper include gloss, fiber coverage, surface smoothness, and overall sheet appearance.
An integral part of any coating, whether it be for printing or another specialty application, is the rheology modifier. The rheology modifier imparts low and high shear viscosity properties to allow application to paper by high speed blade and bar coaters. The rheology modifier is critical in providing uniform application at the desired coating coverage rate. In choosing a rheology modifier, one must take into account possible side effects that may produce undesirable coated paper properties. These additives tend to affect drying rate, gloss, smoothness and porosity. Common rheology modifiers include cellulosic derivatives, polyacrylates, modified starches, and alginates. For example, U.S. Pat. No. 4,341,597 to Andersson et al. ("Andersson") discloses a fibrous material having good dimensional and heat stability--Andersson teaches a coating (e.g., for paper) utilizing polyoxyalkylene. No particular gloss or coat weight benefits are alleged.
Poly(ethylene oxide) has been included in other coating compositions. For example, U.S. Pat. No. 3,892,675 to Clarke et al. ("Clarke et al.") discloses coating compositions with improved whiteness and fastness to light. Clarke et al. discloses coating compositions comprising white pigment extender, polyvinyl acetate latex as the sole binding agent, one or more sparingly water-soluble optical brightening agents in finely-divided form and water. The white pigment extender preferably comprises from 30 to 70% by weight of the coating composition. The optical brightening agent preferably comprises from 0.01 to 0.25% of the weight of the white pigment extender.
As noted in column two, lines 53-63, the coating composition may also include an "assistant" such as polyethylene oxide (or alkyl ether thereof, copolymers of ethylene and propylene oxides, polyvinyl-alcohol, polyvinyl pyrrolidone, and water-soluble condensation products of formaldehyde with urea or melamine). The assistant preferably comprises from 0.002 to 5% of the pigment extender by weight. As discussed in column 3, lines 5-20, Clarke et al. alleges that the coating compositions provide superior whiteness to paper coated therewith. Clarke et al. alleges that the coating compositions may be applied to the paper by conventional means (e.g., air-knife, blade, brush, roller or bar coating techniques). Clarke et al. do not appear to make any allegations about improved gloss through the use of their coatings. Also, Clarke et al. do not appear to disclose any particular molecular weight for their polyethylene oxide.
U.S. Pat. No. 4,169,823 to Jones ("Jones") discloses a coating composition comprising an aqueous emulsion of polyethylene containing a water-soluble polyethylene oxide. Jones teaches the use of a coating composition comprising an aqueous emulsion of a polyethylene (e.g., having an average molecular weight of about 1200 to about 2000), said polyethylene emulsion containing from about 0.5% to about 15% (by weight of the polyethylene emulsion) water-soluble polyethylene oxide. The polyethylene oxide ranges in molecular weight from about 100,000 to about 1,000,000 (col. 3, lines 11-22). The polyethylene oxide-containing polyethylene emulsion may be applied to the substrate (e.g., paper) by conventional means such as dipping, spraying, knife coating, roller coating, air coating, extrusion coating, etc.
Jones alleges that the polyethylene-polyethylene oxide coating provides paper with superior dusting and scuff resistance properties (i.e., relative to conventional coatings--e.g., colloidal silica--see, Table A). No particular improvements in gloss properties of the paper are alleged.
U.S. Pat. No. 5,108,782 to Reed ("Reed") discloses a silicone release coating. Reed teaches a silicone release coating emulsion comprising a reactive crosslinkable silicone, a catalyst and a high molecular weight, water soluble or water dispersible polymeric thickening agent such as polyethylene oxide. The polymeric thickening agent has a molecular weight greater than about 100,000 g/mole Mw. The silicone release emulsion is applied as a coating to a substrate, and particularly paper, and cured to form a release coating for pressure sensitive adhesives. The silicone emulsion exhibits improved holdout of the silicone from the substrate. Reed does disclose (in Table 1) improved paper gloss values with PEO coatings. Nevertheless, Reed does not appear to disclose improved (i.e., higher) coat weights per run through the coating machine.
Until the present invention, it was not known that by using PEO (Poly(ethylene oxide)) it is possible to apply higher coating weights than previously were obtainable with standard coater operating conditions. Likewise, standard coating weights have been hereby found to be obtainable under modified coater operating conditions. This is also beneficial since many specialty coatings can only be run with modified operating conditions. Using PEO as the rheology modifier allows for standard coating weights to be obtained under these modified conditions.
Furthermore, using PEO in the coating formulation, it has been found possible to achieve high coat weights with coatings that have viscosity values that would traditionally be considered too low for high speed coating. Many specialty coating applications require specific surface properties and/or surface chemistry. Laboratory draw downs and bench coating work can be utilized to define the necessary properties for the application in question. However, in order to apply the formulated coating on high speed and high shear coating equipment, a thickener or rheology additive must be included in the formulation. The purpose of the thickener is to raise the high shear and low shear viscosities to a level that will allow for high coat weight application. Typical Brookfield Viscosity might be in the range of 2000 cps. to 5000 cps. Even higher viscosities might be needed for ultra high coat weights (e.g., 10-20 lbs./3300 ft.sup.2).
Unfortunately, the amount of traditional thickener required to achieve ultra high coat weights often changes the surface properties and/or chemistry of the coated sheet. An alternative to using high levels of thickener is to double coat the sheet (two or more coatings on the same side) to achieve the desired paper surface. This can be done with two passes through an off machine coater, with a multi-station coater, or with a precoat applied on the paper machine followed by a single pass through a coater. Each of these methods add additional cost to the coated grade, however.
PEO has been found to provide a coating tool that allows unusually high coating weights to be obtained at very low addition levels of thickener. This insures that a higher percentage of binder/pigment is available on the paper surface for the end use application. Also, this allows the desired paper properties to be obtained with a single coating pass.